Silicon-based solar cells, as mainstream of solar cells at present, exhibit successfully high photoelectric conversion efficiency, but impose large burden on the environment in terms of amount of resources, emission of carbon dioxide during production, and cost etc. In this regard, researches are being conducted worldwide to achieve clean solar cells with less constraint on resources, which may be photoelectric conversion elements using organic semiconductor materials, in particular so-called organic thin-film solar cells configured such that organic semiconductors are interposed between electrodes having different work functions.
In 2005, Heeger et al. made an ITO/PEDOT:PSS/P3HT:PCBM blended film/Al-type cell (referred also to as “normal-type solar cell”, hereinafter), and achieved a photoelectric conversion efficiency (PCE) of 5% (Non-Patent Literature 1).
The “ITO” as used herein is abbreviation of indium tin oxide.
The “PEDOT:PSS” means a polymer compound comprising poly-3,4-ethylenedioxythiophene (PEDOT) and polystyrene sulfonic acid (PSS), which has the structure below.

The “P3HT:PCBM blended film” means a film-like body comprising a mixture of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and respective constituent substances have the structures below.

The currently-reported highest value of photoelectric conversion efficiency in organic thin-film solar cells is 7.4% for a single-type cell, made by Luping Yu et al. in 2010, using a PTB7:PC70BM blended film (Non-Patent Literature 2).
In organic thin-film solar cells, it is particularly important for enhancing the performance of a photoelectric conversion element to transport effectively both positive and negative carriers, generated in the organic semiconductor due to light irradiation, to respective electrodes.
For example, in the above normal-type solar cell, a hole transporting layer comprising the PEDOT:PSS is expected to enhance the hole transport efficiency.
In view of enhancing the transport efficiency for electrons, the present inventors have proposed a technique of interposing an oxide semiconductor that functions as a hole blocking layer between an anode electrode and an organic semiconductor layer (e.g., Patent Literature 1).